Steel pipes such as OCTG (oil country tubular goods including oil well tubing, casing, and drill pipes) used for the exploration and production of oil wells and gas wells, as well as riser pipes, line pipes, and the like are usually connected by a tubular threaded joint.
A tubular threaded joint is constituted by a pin, which is a male threaded element provided on an end portion of a first tubular member, and a box, which is a female threaded element provided on an end portion of a second tubular member. Connection is carried out by engagement of the male screw thread and the female screw thread, which are both tapered screw threads. Typically, the first tubular member is a pipe such as an oil country tubular good, and the second tubular member is a separate member in the form of a coupling (this type of tubular threaded joint is referred to as a coupling type). With this type, a pin is formed on both ends of the pipe, and a box is formed on both sides of the coupling.
There also exist integral-type tubular threaded joints which do not use a coupling and in which a pin is formed on the outer surface of one end of a pipe and a box is formed on the inner surface of the other end of the pipe. With this type, the first tubular member is a first pipe and the second tubular member is a second pipe. In theory, a coupling-type tubular threaded joint in which a pin is formed on a coupling and a box is formed on a pipe is also possible. Below, an explanation will be given primarily of an example of a tubular threaded joint of the type first described above in which a pin is formed on both ends of a pipe and a box is formed on a coupling.
In the past, oil country tubular goods have been connected primarily using standard threaded joints specified by API (American Petroleum Institute) standards. However, in recent years, as the environments for excavation and production of crude oil and natural gas become severe, high-performance special threaded joints referred to as premium joints are being increasingly used.
In a premium joint, the pin and the box each have, in addition to a tapered screw thread, a metal-to-metal seal surface, which makes direct metallic contact in the radial direction between mating members of the joint possible thereby forming a seal, and a torque shoulder surface which serves as an abutting stopper during tightening of the joint.
FIGS. 1(A) and 1(B) are schematic explanatory views of a typical premium joint for oil country tubular goods of the coupling type. FIG. 1(A) is an overall view, and FIG. 1(B) is an enlarged view of a portion thereof. As shown in FIG. 1(B), this tubular threaded joint has a pin 1 which is a male threaded element provided on each end portion of a pipe and a box 2 which is a corresponding female threaded element provided on both sides of a coupling. On its outer surface, the pin 1 has a tapered male threaded zone 11 and an unthreaded generally cylindrical abutting portion 12 called a lip (referred to below as a lip zone) adjacent to the male threaded zone 11 on the side closer to the end. The lip zone 12 has a metal-to-metal seal surface 13 (also referred to below simply as a seal surface) on its outer peripheral surface and a torque shoulder surface 14 (also referred to below simply as a shoulder surface) on its end surface.
The corresponding box 2 has on its inner surface a tapered female threaded zone 21, a metal-to-metal seal surface 23, and a shoulder surface 24 which can interfit with, contact, or abut against the tapered male threaded zone 11, the metal-to-metal seal surface 13, and the shoulder surface 14, respectively, of the pin 1.
FIG. 2 is a schematic view for explaining the shape and dimensions of a trapezoidal thread typified by a buttress thread specified by API. As in FIGS. 1(A) and 1(B), 11 is a male threaded zone and 21 is a female threaded zone. A screw thread used in a premium joint is usually a trapezoidal screw thread modeled on this API buttress screw thread. Most premium joints employ the dimensions of an API buttress screw thread with almost no changes with respect to the aspect ratio (height-to-width ratio), the flank angles (the angle of slope of the flanks), and the like of screw thread.
By way of example, in the case of an API buttress screw thread having a thread pitch of 5 TPI (5 threads per inch), the thread height 74, which is the height to the crest of a male thread, is 1.575 mm, the angle of slope 71 of the load flank is 3°, the angle of slope 72 of the stabbing flank is 10°, and the clearance 73 in the pipe axial direction between the stabbing flanks of the male screw thread and the female screw thread when the load flanks contact each other (the stabbing flank clearance) is an average of approximately 100 μm (30-180 μm).
Concerning the shape of the threads of a tubular threaded joint, WO 92/15815 describes a tubular threaded joint in which the portion connecting the thread crest and the stabbing flank of each thread of both a pin and a box is cut away along a straight line or a curve (namely, it is chamfered) such that it can function as a contact surface which is the first to contact when the pin is inserted into the box. The contact surfaces of the pin and the box are intended to contact each other to facilitate insertion when axial misalignment occurs during insertion of the pin into the box.
U.S. Pat. No. 6,322,110 discloses a tubular threaded joint based on the same concept. Namely, a corner chamfer (chamfer of a corner portion) is provided on the stabbing flanks of the threads of both a pin and a box. When the pin is inserted into the box, the corner chamfers engage with each other and facilitate insertion of the pin.
Above-described WO 92/15815 and U.S. Pat. No. 6,322,110 both facilitate insertion of a pin by preventing misalignment of the insertion angle by producing contact between the pin and the box in the chamfered portions of the stabbing flanks and the crests. Accordingly, chamfered portions are necessary on both the pin and the box, and the intended effect is not exhibited if they are provided on just one of the two members. There is no description in these patent documents concerning the effect of chamfering on resistance to compression.
In a premium joint, a certain amount of interference in the radial direction is provided between the metal-to-metal seal surfaces of a pin and a box. When a joint is tightened until the shoulder surfaces of the pin and the box abut against each other, the seal surfaces of these members are in intimate contact around the entire circumference of the joint, thereby forming a seal.
The shoulder surfaces of the pin and the box function as stoppers which abut at the time of tightening of the joint, and they also have the function of bearing a considerable proportion of the compressive load which acts on the joint. Accordingly, if the thickness of the shoulder surfaces is not large (or if the stiffness of the shoulder surfaces is not high), they cannot withstand a large compressive load.
In the past, vertical wells were predominant, and threaded joints for oil country tubular goods had sufficient performance if they could withstand the tensile load due to the weight of pipes connected thereto and could prevent leakage of high pressure fluid passing through their interior. However, in recent years, the depth of wells has been increasing, sloping wells and horizontal wells in which the well bore bends underground are increasing, and the development of wells in disadvantageous environments such as in the sea or in polar regions is increasing. As a result, a greater variety of performance is being demanded of threaded joints, such as resistance to compression, resistance to bending, sealing ability against external pressure, and ease of use or pin insertion in the field.
When external pressure acts on the above-described conventional premium joint, the applied external pressure is transmitted through gaps between the screw threads and penetrates to the portion just before the seal surfaces shown by 31 in FIG. 1(B). Since the lip zone has a much smaller wall thickness than the body of the pipes being connected, it sometimes undergoes deformation in the form of a reduction in diameter due to the penetrated external pressure. If the external pressure becomes high, a gap may develop between the seal surfaces, leading to the occurrence of leakage, e.g., a situation in which external fluid penetrates into the interior of the pipe body through the gap.
When a compressive load acts on a premium joint, for example, during installation of an oil country tubular good in a horizontal well or a sloping well, since a premium joint usually has a relatively large gap between the stabbing flanks as is the case with the above-described API buttress screw thread, the ability of the screw threads of the joint to bear a compressive load is low, and most of the compressive load is borne by the abutting shoulders thereof.
However, the wall thickness of the shoulder surfaces (the area for receiving compressive loads which corresponds to the area of the lip end surface) is normally considerably smaller than that of the pipe body. Therefore, if a compressive load corresponding to 40-60% of the yield strength of the pipe body is applied, with most premium joints, the lip zone of the pin undergoes a substantial plastic deformation, leading to a marked decrease in the sealing performance of the seal surface adjacent to this portion.
The sealing ability of a joint against external pressure (external pressure sealing ability) can be increased by increasing the stiffness of the pin so as to increase its resistance to deformation toward a reduction in diameter. For this purpose, a technique called swaging is often applied to the pipe towards the axis in order to increase the wall thickness of the lip zone.
However, if the amount of swaging is too great, with casing, there are cases in which a pipe which is inserted into its interior catches on the swaged portion, and with tubing, there are cases in which turbulence develops in a fluid such as crude oil flowing inside the tubing due to the swaged portion and causes erosion. Therefore, the wall thickness of a lip zone can be increased by swaging only to a limited extent.
WO 2004/109173 proposes a tubular threaded joint, as shown in FIG. 3, having a nose portion 15 provided between a metal-to-metal seal surface 13 and a torque shoulder surface 14 on the end surface of a pin 1. The generally cylindrical outer periphery of the nose portion 15 of the pin 1 does not contact the opposing is portion of the box 2. On the other hand, the metal-to-metal seal portions 13 and 23 and the shoulder surfaces 14 and 24 of the pin and the box contact each other. By extending the lip zone of the pin so as to provide a noncontacting nose portion 15 at the end of the seal surface, the wall thickness of the lip zone including the shoulder surface and the seal-surface can achieve a large value within a limited pipe wall thickness, and the resistance to compression and the sealing performance against external pressure of a tubular threaded joint can be markedly increased.